The present invention relates to a hydraulic power transmission joint which is used to distribute a driving force of a vehicle and, more particularly, to a hydraulic power transmission apparatus which relatively rotatably couples two power rotary shafts and transfers a torque corresponding to a difference between the rotational speeds.
Hitherto, as a method of distributing the motive power to front and rear wheels in a four-wheel driving car or distributing the motive power to right and left driving wheels, there has been known a method using a plunger pump mechanism as disclosed in, for instance, U.S. Pat. No. 3,393,583 or U.S. Pat. No. 2,511,518.
However, such a plunger pump mechanism has a structure in which a plurality of plungers are arranged in the radial direction and a liquid pressure is generated by a stroke of the plunger in association with the relative rotation between a cam housing and a rotor. Thus, there are problems such that a cooling efficiency is low, a vibration is large, and an assembling performance is also bad.
Therefore, in U.S. Application Ser. No. 07/461,124, now U.S. Pat. No. 5,037,353, the inventors of the present invention have proposed a hydraulic power transmission joint having a structure such that a plunger is stroked in the direction which is parallel with a shaft in association with the relative rotation between a cam housing and a rotor and a liquid pressure to transfer a torque is generated.
However, in such a conventional hydraulic power transmission joint, since check valves are used as an intake valve and a discharge valve, there is a problem such that a centrifugal force due to the rotation of the joint itself acts on the check valves and the check valves are not closed due to the centrifugal force. It is difficult to realize a joint which correctly functions even in the rotation of a high speed.
On the other hand, in a plunger pump for use in a general industry, a rotary valve is hitherto used. However, the following problems occur in the case of using the rotary valve in a power transmission joint for use in a 4-wheel driving car.
That is, in the conventional rotary valve for use in the general industry, opening/closing phases of the cam to drive the plunger and the valve are fixed and when the rotating direction changes, the relation between the intake and discharge operations for the cam is reversed and the flowing direction of an oil is also reversed. When the flowing direction of the oil is reversed, the discharge oil from a plunger chamber flows to the intake valve side having no orifice, so that no pressure is generated in the plunger chamber and no torque is generated. Consequently, different from the general industrial pump which is used by fixing the rotating direction, the conventional rotary valve cannot be directly used in the power distributing apparatus for use in the 4-wheel driving car in which the differential rotating direction changes such as in the case of the forward and backward movements or the acceleration and deceleration modes.
On the other hand, the rotary valve has a problem such that a force in such a direction as to open a gap of the valve is generated depending on the relation between the liquid pressure and its reaction force and an oil leakage increases so long as a port area of the valve surface is not properly determined.
Further, in the hydraulic power transmission joint using the plunger pump, since the oil is discharged by the reciprocating motion of each plunger, if an air mixedly exists in the operating oil, a volume change of each cylinder is absorbed by the compression of the air and the power transmitting function cannot be effected. Therefore, when the oil is injected into the joint, it is necessary to fill the oil by completely bleeding the air.
There is a method of assembling the parts of the joint in the oil in order to prevent the mixture of the air or the like. However, since the assembling method is difficult or the like, hitherto, two oil injection holes are formed in the central portion of the joint shaft edge and the oil is fed with a pressure from one direction and is discharged from the other direction while giving a differential pressure to the joint, thereby bleeding the internal air.
However, there is a problem such that if the oil injection holes which are closed by screwing tapered plugs after completion of the oil injection are formed in the housing, an outer diameter increases by only an amount of such holes. It is demanded to form the oil injection holes to proper positions such as not to increase the joint.